1. Field of the Invention
The present invention relates to a semiconductor device basically made of silicon carbide and a module device incorporating the semiconductor device. In this specification, the present invention is discussed, taking a case of high-voltage semiconductor device as an example, but the present invention is not limited to such an application and can be applied to semiconductor devices for various uses.
2. Description of the Background Art
As well known, a current inverter needs voltage blocking capability of reverse direction. Therefore, a current inverter which has a switching element without voltage blocking capability of reverse direction (e.g., IGBT and power MOSFET) generally uses a diode connected in series to the switching element.
FIG. 7 is a vertical cross section showing a background-art module element used for a current inverter. As shown in FIG. 7, a switching device 601 and a diode 602 both of which are basically made of silicon are provided in an encapsulating container 617.
The switching device 601 has a cathode electrode 603 and a control electrode 604 formed on a surface thereof and an anode electrode 605 formed on a back surface thereof. The diode 602 has an anode electrode 606 formed on a surface thereof and a cathode electrode 607 formed on a back surface thereof. The anode electrode 605 of the switching device 601 and the cathode electrode 607 of the diode 602 are both soldered onto a conducting board 608. Therefore, these electrodes 605 and 607 are electrically connected to each other with a solder layer 609 and the conducting board 608 interposed therebetween. Contrary to this, the cathode electrode 603 and the control electrode 604 of the switching device 601 are connected to a cathode conducting bar 610 and a control conducting bar 611 with bonding wires 613, respectively. The anode electrode 606 of the diode 602 is connected to an anode conducting bar 612 with a bonding wire 613.
On the other hand, the conducting board 608 is connected to a metal body 615 comprising a hollow trench 616 with an insulating substrate 614 interposed therebetween. In the trench 616, a cooling medium such as water is circulated. With such a structure, heat generated by loss of the switching device 601 and the diode 602 is radiated through the electrodes 605 and 607, the solder layer 609, the conducting board 608, the insulating substrate 614, the metal body 615 and the cooling medium which are formed on the back surfaces thereof.
Silicon carbide, which has larger energy gap between bands than silicon, is highly thermally stable. Therefore, a device using silicon carbide is operable even at a high temperature up to 1000 Kelvin. Moreover, since silicon carbide has larger thermal conductivity than silicon, silicon carbide devices can be arranged at high densities. Further, silicon carbide, which has breakdown voltage about ten times as large as silicon, is suitable to be used as a base material for a device which operates under a condition that high voltage may be generated in a blocking state of the device. In other words, the thickness of a silicon carbide device needed to maintain a certain level of voltage may be significantly thinner than that of a device whose base material is silicon, and therefore it is expected that the silicon carbide device can resolve an antinomic relation between a switching loss and a steady-state loss.
Since silicon carbide has breakdown voltage about ten times as large as silicon, in a silicon carbide device, the width of a depletion layer needed for a certain level of voltage blocking capability is very small. Therefore, the distance between an anode electrode and a cathode electrode is short and accordingly the voltage drop in a current-carrying state which is almost proportional to the electrode distance becomes small. In other words, use of silicon carbide as a base material makes it possible to reduce the steady-state loss caused in the current-carrying state. With this effect, a switching device or a diode using silicon carbide has an advantage of significantly resolving the antinomic relation between the switching loss and the steady-state loss as compared with a switching device or a diode using silicon.
Further, the silicon carbide device, which can operate at a high temperature, has an advantage of simplifying a device cooling system such as a heat sink.
Forming a pn junction in silicon carbide, however, needs heat treatment of much higher temperature than forming a pn junction in silicon and can not disadvantageously use an existing manufacturing facility for silicon.
Furthermore, when a wire is ultrasonically bonded onto an electrode formed on a surface of silicon carbide, a stress generated depending on the conditions such as a load in bonding is applied to the electrode and this changes a condition of junction between the silicon carbide and the electrode, thereby disadvantageously not producing an expected performance.
Moreover, in a silicon carbide device having a conducting board which is in electrical contact with silicon carbide, when there is difference in thermal expansion coefficient between the silicon carbide and the conducting board, a stress caused by heat cycle changes the performance of the device.
The present invention is directed to a semiconductor device. According to a first aspect of the present invention, the semiconductor device comprises: a silicon carbide layer of predetermined conductivity type comprising a surface having a first region, a second region and a third region sandwiched between the first region and the second region; an anode electrode having a Schottky contact with the first region; a cathode electrode having an ohmic contact with the second region; and a control electrode having a Schottky contact with the third region.
According to a second aspect of the present invention, in the semiconductor device of the first aspect, at least one Schottky barrier electrode out of the anode electrode and the control electrode has a thickness of not less than 5 xcexcm.
According to a third aspect of the present invention, in the semiconductor device of the first aspect, the silicon carbide layer further comprises a back surface opposite to the surface, and the semiconductor device of the third aspect further comprises: a semi-insulation substrate formed on the back surface of the silicon carbide layer; and a metal layer formed on a surface of the semi-insulation substrate.
The present invention is also directed to a module device. According to a fourth aspect of the present invention, the module device comprises: a conducting board; the semiconductor device as defined in the third aspect having the metal layer formed on a surface of the conducting board with a solder layer interposed therebetween; and an encapsulating container for encapsulating the conducting board and the semiconductor device, and in the module device of the fourth aspect, only the semiconductor device is formed on the surface of the conducting board.
The present invention is still directed to a semiconductor device. According to a fifth aspect of the present invention, the semiconductor device comprises: a silicon carbide layer of predetermined conductivity type; and a Schottky barrier electrode having a Schottky contact with a predetermined region on a surface of the silicon carbide layer, and in the semiconductor device of the fifth aspect, the Schottky barrier electrode has a thickness of not less than 5 xcexcm.
According to a sixth aspect of the present invention, the semiconductor device comprises: a silicon carbide layer of predetermined conductivity type; a plurality of electrodes formed on a surface of the silicon carbide layer; and a substrate being brought into electrical contact with at least one electrode out of the plurality of electrodes by an external pressure, and in the semiconductor device of the sixth aspect, the substrate is basically made of any one of carbon, silicon carbide, aluminum, gold, silver and copper.
According to a seventh aspect of the present invention, in the semiconductor device of the sixth aspect, the at least one electrode is a Schottky barrier electrode, and the Schottky barrier electrode has a thickness of not less than 5 xcexcm.
According to an eighth aspect of the present invention, in the semiconductor device of the seventh aspect, the plurality of electrodes include the Schottky barrier electrode formed on a first surface of the silicon carbide layer and an ohmic electrode formed on a second surface of the silicon carbide layer opposite to the first surface, and the semiconductor device of the eighth aspect further comprises: a first substrate being brought into electrical contact with the Schottky barrier electrode by an external pressure; and a second substrate being brought into electrical contact with the ohmic electrode by an external pressure, and in the semiconductor device of the eighth aspect, the first substrate is basically made of any one of carbon, silicon carbide, aluminum, gold, silver and copper, and the second substrate is basically made of any one of carbon, silicon carbide, aluminum, gold, silver and copper.
By the first aspect of the present invention, it is possible to provide a semiconductor device with high breakdown voltage and low loss, which has both a switching function and a diode function (voltage blocking capability of reverse direction) with no pn junction formed therein and eliminate the necessity of serially connected diodes which are needed in the background art.
In the second aspect of the present invention, since a stress applied to at least one Schottky barrier electrode when wire is bonded onto the Schottky barrier electrode can be effectively dispersed and relieved, it is possible to achieve a highly reliable semiconductor device which causes little change in performance at the interface of Schottky contact.
In the semiconductor device of the third aspect of the present invention, it is possible to achieve a switching element of high breakdown voltage with low switching loss and low steady-state loss, which has voltage blocking capability of reverse direction, on the semi-insulation substrate.
In the fourth aspect of the present invention, since there is no necessity of providing any diode in the encapsulating container, it is possible to ensure reduction in size and weight of a module device.
In the fifth aspect of the present invention, a stress applied to the Schottky barrier electrode when wire is bonded onto the Schottky barrier electrode can be effectively dispersed and relieved to prevent a change in level of Schottky barrier and performance and it thereby becomes possible to provide a highly reliable semiconductor device.
In the sixth aspect of the present invention, since the substrate can relieve a stress due to heat cycle, it is possible to provide a highly reliable semiconductor device which is unlikely to cause a change in performance.
In the seventh aspect of the present invention, since the Schottky barrier electrode itself can effectively disperse and relieve a stress applied thereto when the Schottky barrier electrode is externally pressed, it is possible to provide a semiconductor device of much higher reliability, which is more unlikely to cause a change in performance.
Since the semiconductor device of the eighth aspect of the present invention is unlikely to cause a change in performance by either the stress due to the heat cycle or the stress caused by external pressure, it is possible to achieve a diode using a silicon carbide layer, which has high reliability.
An object of the present invention is to improve performance of a semiconductor device using silicon carbide as a base material.
More specifically, a first object of the present invention is to develop a silicon carbide device with low loss, having both a switching function and a diode function, to ensure reduction in size and weight of a module device.
Further, a second object of the present invention is to develop a silicon carbide device of high reliability which is unlikely to cause a change in performance even when a stress caused by wire bonding or heat cycle is applied thereto.